f-ratio (oceanography)

The ratio was originally defined by Richard Eppley and Bruce Peterson in one of the first papers estimating global oceanic production.

[1] This fraction was originally believed significant because it appeared to directly relate to the sinking (export) flux of organic marine snow from the surface ocean by the biological pump.

However, this is a relatively difficult process, since traps can be awkward to deploy or recover, and they must be left in situ over a long period to integrate the sinking flux.

By measuring how much nitrate is consumed by primary production, relative to that of regenerated ammonium, one should be able to estimate the export flux indirectly.

Indeed, in their original paper, Eppley & Peterson noted that: "To relate new production to export requires that nitrification in the euphotic zone be negligible.

After including nitrification measurements in its parameterisation, an ecosystem model of the oligotrophic subtropical gyre region (specifically the BATS site) found that, on an annual basis, around 40% of surface nitrate was recently nitrified (rising to almost 90% during summer).

For this reason, some workers have proposed distinguishing between the f-ratio and the ratio of particulate export to primary production, which they term the pe-ratio.

In addition, a further process that potentially complicates the use of the f-ratio to estimate "new" and "regenerated" production is dissimilatory nitrate reduction to ammonium (DNRA).

Since, like nitrification, DNRA alters the balance in the availability of nitrate and ammonium, it has the potential to introduce inaccuracy to the calculated f-ratio.

However, as DNRA's occurrence is limited to anaerobic situations,[14] its importance is less widespread than nitrification, although it can occur in association with primary producers.